1. Field of the Invention
The present invention relates generally to fabrication of photovoltaic semiconductor devices in which optical confinement features increase photo conversion efficiency and more particularly to texturing of surfaces to enhance optical confinement in the semiconductor device.
2. Description of the Prior Art
Photovoltaic semiconductor devices that capture incident electromagnetic radiation, such as light radiation from the sun or other light sources, and convert the radiation to electricity are more efficient converters if more of the incident light can be absorbed in the crystal lattice and converted to electricity, instead of merely passing entirely through the device, being reflected back out of the device, or being absorbed, but converted to heat instead of electricity. Antireflective coatings are used to increase transmittance of incident light into the semiconductor material, and it is know that textured surfaces can scatter light in the semiconductor device to decrease likelihood of transmittance of the light all the way through or loss by reflection off the back surface and then transmittance out the front surface. Therefore, surface texturing has been used to reduce surface reflectance and to produce broad-band antireflection (AR) characteristics as well as to produce optical confinement by scattering light in a way that ensures multireflection with total internal reflections at cell surfaces or interfaces. However, texturing processes and results have not been entirely satisfactory prior to this invention, as will be described below.
Several techniques have been developed prior to this invention for texturing surfaces of semiconductor devices, including masked chemical etching, unmasked chemical etching, and laser grooving. Front surface etching has been used to reduce surface reflectance and to scatter incoming, normally-incident light at oblique angles within the device. However, a disadvantage of front side texturing is that it causes an increased dark current, hence a lower open circuit voltage and fill factor, i.e., lower quality and less efficient photovoltaic cell. Also, texturing on the junction side of a semiconductor device, which is usually, but not necessarily always, on the front side, with previously used techniques, can result in textured peaks that break the junction and cause shunting, thus serious damage to or destruction of the device. Therefore, backside texturing is preferred over front side texturing, if other antireflective measures, such as AR coatings, are used on the front surface.
Backside texturing, however, also has had its problems, one of the most significant of which is that the backside is usually used for the metal electrical contact. Thus, backside texturing in previously used processes involves chemical etching or otherwise texturing the back surface prior to deposition of the metal contact layer, which requires masking the front surface to avoid damage during the backside etching process. The device then has to be thoroughly cleaned before deposition of the metal contact layer. Since conventional deposition processes, such as vapor deposition, sputtering, or electrolytic precipitation deposit material uniformly, the outside surface of the metal contact layer will also be textured corresponding to the textured semiconductor surface. Also, the initial semiconductor/metal interface, while bonded in the deposition process, still has a high resistivity that must be lowered substantially, either by alloying or sintering, in order to be useful as an electric contact for the photovoltaic device. Alloying generally creates a better bond and a lower resistivity interface, although sintering is often used, especially on the side of the junction, to avoid other problems such as damage to the junction, that can result from conventional alloying techniques. However, conventional alloying, even with the newer rapid thermal alloying (RTA) techniques, is difficult to control and tends to create a deep, graded alloy layer at the semiconductor/metal interface that absorbs light and dissipates the energy as heat, instead of reflecting the light back into the semiconductor material where it can be absorbed and convened to electricity.